15,480 research outputs found
Noise spectrum of photographic emulsions
Thesis (M.A.)--Boston UniversityIt is the purpose of this thesis to measure the energy spectrum, referred to in this thesis as the noise spectrum, of the granularity of photographic emulsions. This was accomplished by using an optical diffraction device as a spatial spectrum analyzer. A uniformly exposed developed emulsion is illuminated by a plane wave, and the diffraction pattern obtained gives the grain energy distribution; i. e., the noise spectrum, as a function of spatial frequency.
The analyzer is composed of a point source which coherently illuminates a uniformly exposed developed emulsion in the aperture of a positive lens, with monochromatic light, and a device for photographing the diffraction pattern at the Fraunhofer plane.
The targets were chosen so as to cover the films commercially available and also obtain data on coarse, medium and fine grain emulsions. These films were Royal X Pan, Super XX and Micro-File (5402). Three uniform average densities of 0.28, 1. 00, and 1. 76 of each film type were used.
These targets were placed in the target plane (lens aperture) and a picture of the diffraction pattern was taken by placing a sheet of Royal Pan film in the Fraunhofer plane. A picture was taken of the diffraction pattern of each of the nine targets. These pictures were then reduced to density vs. distance traces then converted to energy distribution curves.
Some inherent reflected and stray light was discovered present in the analyzer. This reflected and stray light was caused by lens surface reflections and low quality lens surfaces. The energy spectrum of the granularity noise was reduced by the amount of this inherent reflected and stray light which was transmitted through the target.
A historical development of previous work done on the problem is presented in two parts, the earlier approaches, and the later application of Communication Theory to this problem. Then the new approaches to the problem developed as a result of this Communication Theory outlook on optics are discussed. The analytical considerations of the Fourier approach to the problem used in this thesis are also discussed.
The results of this investigation indicated that the fine grain emulsion had a narrow noise spectrum as compared to the coarse grain emulsion. This proved to be in direct opposition to theory. The theory is presented in the appendix. It was finally concluded that the approach used in this case is invalid because the inter-reflections between the multiple grain layers in the emulsion destroy the true diffraction pattern. A means of verifying this conclusion is also presented
Neutron-Capture Nucleosynthesis in the First Stars
Recent studies suggest that metal-poor stars enhanced in carbon but
containing low levels of neutron-capture elements may have been among the first
to incorporate the nucleosynthesis products of the first generation of stars.
We have observed 16 stars with enhanced carbon or nitrogen using the MIKE
Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the
Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present
radial velocities, stellar parameters, and detailed abundance patterns for
these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and
other heavy elements are detected. In four stars, these heavy elements appear
to have originated in some form of r-process nucleosynthesis. In one star, a
partial s-process origin is possible. The origin of the heavy elements in the
rest of the sample cannot be determined unambiguously. The presence of elements
heavier than the iron group offers further evidence that zero-metallicity
rapidly-rotating massive stars and pair instability supernovae did not
contribute substantial amounts of neutron-capture elements to the regions where
the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor
stars with low levels of neutron-capture elements were enriched by products of
zero-metallicity supernovae only, then the presence of these heavy elements
indicates that at least one form of neutron-capture reaction operated in some
of the first stars.Comment: Accepted for publication in the Astrophysical Journal (36 pages, 26
figures
Kappa symmetry, generalized calibrations and spinorial geometry
We extend the spinorial geometry techniques developed for the solution of
supergravity Killing spinor equations to the kappa symmetry condition for
supersymmetric brane probe configurations in any supergravity background. In
particular, we construct the linear systems associated with the kappa symmetry
projector of M- and type II branes acting on any Killing spinor. As an example,
we show that static supersymmetric M2-brane configurations which admit a
Killing spinor representing the SU(5) orbit of are generalized
almost hermitian calibrations and the embedding map is pseudo-holomorphic. We
also present a bound for the Euclidean action of M- and type II branes embedded
in a supersymmetric background with non-vanishing fluxes. This leads to an
extension of the definition of generalized calibrations which allows for the
presence of non-trivial Born-Infeld type of fields in the brane actions.Comment: 9 pages, latex, references added and minor change
Strategy towards Mirror-fermion Signatures
The existence of mirror fermions interacting strongly under a new gauge group
and having masses near the electroweak scale has been recently proposed as a
viable alternative to the standard-model Higgs mechanism. The main purpose of
this work is to investigate which specific experimental signals are needed to
clearly differentiate the mirror-fermion model from other new-physics models.
In particular, the case is made for a future large lepton collider with c.o.m.
energies of roughly 4 TeV or higher.Comment: 30 Latex pages, 2 postscript figure
Capsaicin-Induced Ca2+ Influx and Constriction of the Middle Meningeal Artery
Research in the past on transient receptor potential cation channel subfamily V member 1 (TRPV1) has been limited to mainly nervous tissue TRPV1 because of the channel’s role in pain perception. Here, we studied the potential role of TRPV1 in vascular smooth muscle. We have observed that capsaicin, a TRPV1 agonist, induced constriction of the middle meningeal artery (MMA). Our goal was to decipher the mechanism of capsaicin-induced constriction of the MMA. Arterial diameter measurements showed that constriction due to 100 nM capsaicin (65.4% ± 3.7, n=7) was significantly diminished in the presence of the voltage-dependent calcium channel (VDCC) blocker 100 µM diltiazem (43.1% ± 8.1, n=7). Capsaicin-induced constriction was not significantly altered in the presence of the sarco/endoplasmic reticulum calcium transport ATPase (SERCA) inhibitor 30 µM cyclopiazonic acid (63.7 ± 9.0%, n=5) compared to control arteries (58.4 ± 8.6%, n=5). The unaltered capsaicin-induced constriction of the MMA in the presence of a SERCA inhibitor suggests that calcium-induced calcium release does not contribute to the overall calcium influx mechanism within the smooth muscle cells of the MMA. The diminished capsaicin-induced constriction of the MMA in the presence of a VDCC blocker suggests that sodium entry through TRPV1 channels can possibly lead to the membrane potential depolarization and increased activity of VDCCs causing further calcium influx. Furthermore, since the capsaicin effect was not abolished by the blockage of VDCCs, our data suggest that calcium entry through TRPV1 is sufficient to cause approximately 65% of the total constriction of the MMA in response to activation of TRPV1
Supersymmetric geometries of IIA supergravity I
IIA supergravity backgrounds preserving one supersymmetry locally admit four
types of Killing spinors distinguished by the orbits of on the
space of spinors. We solve the Killing spinor equations of IIA supergravity
with and without cosmological constant for Killing spinors representing two of
these orbits, with isotropy groups and .
In both cases, we identify the geometry of spacetime and express the fluxes in
terms of the geometry. We find that the geometric constraints of backgrounds
with a invariant Killing spinor are identical to
those found for heterotic backgrounds preserving one supersymmetry.Comment: 21 page
NETCU: analising e-Learning neworked curricula in Europe: the importance of legal and quality assurance aspects
Conferência realizada no Porto de 6-9 de junho de 2012info:eu-repo/semantics/publishedVersio
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